This invention relates generally to the field of devices for protecting electrical circuits in the event of faults, and more particularly to a device for protecting a circuit against an arc fault employing an asymmetrical transformer.
Ground fault circuit interrupters have become quite widely used and they provide the very useful function of disconnecting an electrical power source from a load when a ground fault is detected. Among the more common types of ground faults sensed by known ground fault circuit interrupters are those caused when a person accidentally makes contact with a hot electrical lead and ground. In the absence of a ground fault circuit interrupter, life threatening amounts of currents could flow through the body of the person.
Virtually all ground fault circuit interrupters use a differential current transformer for sensing ground faults. The current transformer is arranged with the hot and neutral leads of an electrical circuit passing through the center of a transformer, preferably a toroidal transformer with a symmetrically wound secondary formed thereon. In normal operation of an electrical circuit, the currents flowing through the hot electrical conductor and neutral electrical conductor are equal and opposite, so that no output signal is produced by the differential transformer. When a ground fault occurs, the currents are no longer the same, so the differential transformer produces a signal that can be sensed by appropriate signal conditioning circuitry to activate a relay or contactor or the like to interrupt the electrical circuit.
Ground fault circuit interrupters must also deal with fundamentally different types of faults that occur when the neutral conductor is connected to the ground conductor in the downstream circuit powered by the ground fault circuit interrupter, where the neutral and ground connections are intentionally but wrongly connected together, or by accidental short circuits formed, for example, when one strand of a stranded electrical conductor accidentally bridges the neutral and ground connections.
If a xe2x80x9cgrounded neutralxe2x80x9d fault of the type just discussed occurs in a circuit in which the ground and neutral lines are connected together, and a person inadvertently makes contact with a hot electrical lead while also connected to the grounded neutral fault, the return current is divided between the neutral electrical conductor and the ground conductor. Only the neutral conductor passes through the differential transformer, so that only a fraction of the ground fault current is available to be sensed. The neutral electrical conductor can be a larger gauge wire than the ground conductor, and does not include resistive connections such as in conduit grounds; therefore, the larger portion of the ground fault current often flows in the neutral wire. In a circuit where a 6 milliamp ground fault current flows through a person, for example, it may result in three-quarters of the current flowing through the neutral wire where it is seen as a load current and therefore not detected, with only one quarter flowing through the ground conductor. Therefore, a much larger ground fault current must flow before the fault is detected, all to the detriment of the person through whom the ground fault current flows.
The problem just mentioned has been commonly addressed by providing a second transformer, sometimes referred to as a grounded neutral transformer. The second transformer is arranged with the hot and neutral lines extending through the core of the transformer, forming a first winding, with another winding wound on the toroidal core forming the second winding. Rather than sensing differential current through the second winding wound on the toroidal core, however, an oscillator is connected thereto, with the second winding of the grounded neutral transformer forming a part of the resonant circuit of the oscillator. In the absence of a ground neutral connection, there is insufficient feedback in the oscillator to initiate and sustain oscillation. However, when a grounded neutral fault occurs, it forms a closed coupling loop between differential and neutral transformers, creating a feedback path and initiating oscillation. The oscillation induces a current in the neutral lead that is detected in the same manner as a ground fault by the primary differential transformer.
Another approach to sensing grounded neutral faults is to provide a transformer through which only the neutral line of the electrical circuit passes. However, if a grounded neutral transformer is provided that has only the neutral line passing through it, it is not able to sense grounded neutral conditions that arise when an electrical circuit is inadvertently connected to the ground fault circuit interrupter with the hot and neutral cable wires reversed. Therefore, the grounded neutral transformer is preferably arranged with both the hot and neutral lines passing through the neutral transformer and forming two secondaries thereof. An arc fault cannot be readily sensed at such a transformer, and a third transformer is ordinarily provided for detecting arc faults, with the primary being only one of the hot and neutral leads.
Ground fault circuit interrupters of the types described detect both conventional ground faults and ground faults in the presence of intentional or accidental grounded-neutral faults. It is desirable to provide circuitry for detecting arc faults as well as ground faults. Arc faults are typically undetectable by the differential transformer or the grounded neutral transformer of a ground fault circuit interrupter because the wave forms produced by an arc fault appear on both the hot and neutral lines.
Because a differential ground fault sensing transformer must be as symmetrical as possible to reduce common mode response, signals indicating arc faults cannot be sensed from the secondary winding of the differential transformer.
The need for three transformers, a primary differential transformer for sensing ground faults, a grounded neutral transformer, and an arc fault sensing transformer, creates a particular problem. Often there is simply not enough room for all three transformers and their associated circuitry to be included in a package that fits in the space provided for a duplex receptacle.
Briefly stated, a protection device for an electrical circuit includes an asymmetrical transformer for producing a signal in response to an arc fault in the circuit; and an arc fault detector connected to the transformer which receives the signal. The asymmetrical transformer preferably includes a primary winding of the asymmetrical transformer being formed by a hot conductor and a neutral conductor of the electrical circuit. An asymmetry of a symmetrical secondary winding is produced by placing an asymmetrical tap into the symmetrical secondary winding of the transformer. The tap receives an asymmetrical part of common mode core flux to produce the signal, while the symmetrical secondary winding receives full common mode flux but in equal and opposite directions so as not to produce a signal.
According to an embodiment of the invention, a protection device for an electrical circuit includes an asymmetrical transformer for producing a signal in response to an arc fault in the circuit; and an arc fault detector connected to the transformer which receives the signal.
According to a feature of the invention, the asymmetrical transformer includes a primary winding of the asymmetrical transformer being formed by a hot conductor and a neutral conductor of the electrical circuit; a symmetrical secondary winding of the transformer; wherein an asymmetry of the symmetrical secondary winding is produced by placing an asymmetrical tap into the symmetrical secondary winding of the transformer, such that the tap receives an asymmetrical part of common mode core flux to produce the signal, whereas the symmetrical secondary winding receives full common mode flux but in equal and opposite directions so as not to produce a signal.